1. Technical Field
This invention relates to a wide field of view optical system and, more particularly, to an all-reflective focal or image forming three-mirror anastigmat optical system.
2. Discussion
Wide field of view optical systems, such as reflective telescopes are utilized in multiple spectral applications and in certain space sensor systems. For applications in which navigation, pilotage, or driving of a vehicle are involved, the optical system must provide a large two-dimensional field of view such as 20.times.30, 20.times.40, or 30.times.40 degrees. It is believed that in future pilotage, navigation, and driving sensors that scanned linear detector arrays will be replaced by full two-dimensional staring arrays. The optical and mechanical simplification in the evolution of staring arrays is apparent. Existing platinum-silicide staring arrays are believed to be but a temporary step in the evolution process. It is believed that future staring arrays will be higher quantum efficiency mercury-cadmium-telluride staring arrays.
Current existing all-reflective optical systems are generally of two types. One type is the relayed design which provides needed cold shielding, however, the shielding is only provided for high aspect ratio line fields of view. Another type of system is a nonrelayed design which provides needed wide two-dimensional fields of view, however, these systems do not provide the required cold shielding.
Existing all-reflective relayed focal telescopes which provide detector cold shielding are illustrated by U.S. Pat. Nos. 4,101,195 issued Jul. 18, 1978 to Korsch, entitled "Anastigmatic Three-Mirror Telescope"; 4,265,510 issued May 5, 1981 to Cook, entitled "Three-Mirror Anastigmatic Optical System"; and 4,834,517 issued May 30, 1989 to Cook, entitled "Method and Apparatus for Receiving Optical Signals", both of the Cook patents are assigned to the same assignee of the present invention, all of the specifications of which are expressly incorporated by reference. While these patents provide desired detector cold shielding, they are limited to two-dimensional fields of view significantly below that needed in pilotage, navigation, or driving operations. Generally, these patents provide high aspect ratio line fields of view which are utilized with current scanning sensors.
The art also illustrates all-reflective, non-relayed, focal three-mirror telescopes which provide generally larger fields of view than the above cited references and in some cases the fields of view are sufficient for pilotage, navigation, or driving. Such patents are illustrated by U.S. Pat. Nos. 4,240,707 issued Dec. 23, 1980 to Wetherell et al., entitled "All-reflective Three Element Objective"; and 4,733,955 issued Mar. 29, 1988 to Cook, entitled "Reflective Optical Triplet Having a Real Entrance Pupil", which is assigned to the assignee of the present invention, both specifications of which are herein expressly incorporated by reference. While these patents illustrate applications with wide fields of view, these non-relayed designs fail to provide the needed detector cold shielding.
Also, various types of refractive systems have been utilized which provide large field of view capabilities, however, these refractive systems have several disadvantages. Some limitations of refractive telescopes are chromatic aberrations, spectral limitations (e.g., visible TV system cannot share the same telescope as the LWIR FLIR), defocus with temperature change requiring compensation, potentially high narcissus, and high cost associated with the complexity and expensive refractive materials.